A unit as disclosed for example in Japanese Patent Application Publication No. 2009-190440, configured as an in-wheel motor unit, has heretofore been proposed as such a motor drive unit. The in-wheel motor unit uses an annular coupling member to couple an output end of a motor drive system and a wheel hub (or a rotator) rotatably supported with a hub bearing (or a bearing unit).
In an electric vehicle including such an in-wheel motor unit for each drive wheel, when an electric motor is driven, rotation of the electric motor is transmitted from the output end of the motor drive system via the annular coupling member to the wheel hub (or the drive wheel) thereby to allow the vehicle to travel.
Incidentally, wearing of the hub bearing (or the bearing unit) in itself supporting rotatably the wheel hub (or the drive wheel) as the rotator, deflection of the wheel hub (or the drive wheel), or the like may cause the wheel hub (or the drive wheel) to be displaced relative to the output end of the motor drive system.
Such a displacement of the wheel hub (or the drive wheel), if affecting the output end of the motor drive system, causes a change in an air gap between a stator and a rotor of the electric motor and hence imposes performance deterioration or torque variations in the electric motor, or, if a reduction gear mechanism is interposed in the output end of the motor drive system, causes contact of teeth of the reduction gear mechanism and hence imposes gear noise or a reduction in gear longevity and further an increase in power loss due to a change in a backlash between gears.
Therefore, Japanese Patent Application Publication No. 2009-190440 takes a measure so as to prevent the displacement of the wheel hub (or the drive wheel) from affecting the output end of the motor drive system. The measure is such that an annular coupling member is additionally provided in an abutting portion of the output end of the motor drive system and a shaft of the wheel hub (or the drive wheel), the annular coupling member and the output end of the motor drive system are drivingly coupled together in a relatively displaceable manner and a motor-side coupler is set between the annular coupling member and the output end of the motor drive system, and the opposite end of the annular coupling member and the shaft of the wheel hub (or the drive wheel) are drivingly coupled together in a relatively displaceable manner and a wheel-side coupler is set between the opposite end of the annular coupling member and the shaft of the wheel hub (or the drive wheel).
The motor-side coupler and the wheel-side coupler are set for the purpose of serving their respective coupler functions to accommodate the displacement of the wheel hub (or the drive wheel) and thereby prevent the displacement of the wheel hub (or the drive wheel) from reaching the output end of the motor drive system.
However, in the above-described conventional motor drive unit, the motor-side coupler and the wheel-side coupler are set on ends of the annular coupling member and are arranged adjacent to each other in the axial direction, and a center of displacement of the wheel hub (or the drive wheel) defined by the hub bearing (or the bearing unit) supporting rotatably the wheel hub (or the drive wheel), therefore, the hub bearing (or the bearing unit), is located farther from the output end of the motor drive system than the annular coupling members, or than the motor-side coupler and the wheel-side coupler.
Thus, the motor-side coupler, the wheel-side coupler, and the center of displacement of the wheel hub (or the drive wheel) are sequentially arranged in this order in the axial direction, and the following problem arises.
The wearing of the hub bearing (or the bearing unit) in itself or the deflection of the wheel hub (or the drive wheel) may cause swinging displacement of the wheel hub (or the drive wheel) about the center of displacement defined by the hub bearing (or the bearing unit).
Therefore, in the conventional motor drive unit in which the motor-side coupler, the wheel-side coupler, and the center of displacement of the wheel hub (or the drive wheel) are sequentially arranged in this order in an axial direction, only an extremely slight amount of swinging displacement about the center of displacement of the wheel hub (or the drive wheel), within a tiltable range of the annular coupling member, can be accommodated, and most of the swinging displacement about the center of displacement of the wheel hub (or the drive wheel) cannot be accommodated.
Thus, the following problem arises. Specifically, transmission of the swinging displacement about the center of displacement of the wheel hub (or the drive wheel), from the output end of the motor drive system to the reduction gear mechanism and via the reduction gear mechanism to the electric motor cannot be avoided. Thus, the gear noise or the reduction in the gear longevity due to the contact of the teeth of the reduction gear mechanism and the increase in the power loss due to the change in the backlash between the gears cannot be prevented as intended, and the performance deterioration or the torque variations in the electric motor due to the change in the air gap in the electric motor cannot be prevented as intended.